Interest in photovoltaic cells, also known as solar cells, has grown rapidly in the past few decades. Photovoltaic cells comprise semiconductor junctions such as p-n junctions. It is known that light with photon energy greater than the band gap of an absorbing semiconductor layer in a semiconductor junction is absorbed by the layer. Such absorption causes optical excitation and the release of free electrons and free holes in the semiconductor. Because of the potential difference that exists at a semiconductor junction (e.g., a p-n junction), these released holes and electrons move across the junction in opposite directions and thereby give rise to flow of an electric current that is capable of delivering power to an external circuit. The flow of carriers into the external circuit constitutes an electrical current density, J amp cm-2, which, under short-circuit conditions, is known as the short-circuit current density, Jsc. At the same time, the separation of the charges (holes and electrons) sets up a potential difference between the two ends of the material, φ, which under open circuit conditions is known as the open-circuit voltage, φoc. It is desirable to maximize both Jsc and φoc. For interaction with the solar spectrum, Jsc and φoc are optimized when the junction semiconductor absorber has a band gap of about 1.4 electron volts (eV).
It is presently common practice to provide an array of photovoltaic cells to generate electrical energy from solar radiation. Many photovoltaic cells are made of silicon. However, cells made of other materials, e.g., cadmium sulfide and gallium arsenide, have also been developed and tested. Crystalline silicon has traditionally been a favored material since it has a band gap of approximately 1.1 eV and thus favorably responds to the electromagnetic energy of the solar spectrum. However, because of the expense in making crystalline silicon-based cells, thin film photovoltaic cells made of materials other than silicon have been explored and used.
Presently, many photovoltaic cells are fabricated as separate physical entities with light gathering surface areas on the order of 4-6 cm2 or larger. For this reason, it is standard practice for power generating applications to mount the cells in a flat array on a supporting substrate or panel so that their light gathering surfaces provide an approximation of a single large light gathering surface. Also, since each cell itself generates only a small amount of power, the required voltage and/or current is realized by interconnecting the cells of the array in a series and/or parallel matrix.